I recently washed my Shoei multitec (for the first time)
1) Remove the cheekpads
2) Remove the chin-strap liners
3) Remove the visor
Then i filled up the sink with luke wam water, added some
gentle shampoo (the kind you use to hand wash delicate clothes) and submerged the helmet - while giving it a rather enjoyable (gentle) scrubbing

Cheekpads and chin liners got the same treatment...
Basically, the helmets interior is only dirty from sweat & salt - both of which are easily cleaned away by mild soap/shampoo.
Once done, i refilled the sink with clean water (no soap or shampoo) and washed the helmet (+pads & straps) until all the soapy residue had dissapeared.
I dried the exterior with a soft towel and left the helmet outside (away from the sun)
By morning time (its REALLY HOT over here

) my helmet was completely dry and smelled fantastic.
So, wash you helmet. Your face & nose will thank you
